An OTR technician at a quarry installed two spare tires and a new wheel on a Komatsu WA900 loader. While demounting the scrap tire/wheel in the storage yard, EE removed the O-ring with EE's left hand. Residual pressure caused the bead seat band to spring upward, striking EE's left thumb and index finger. This amputated the tip of EE's left index finger and crushed EE's left thumb.